sd-event: introduce concept of "floating" event sources

These are the counterpart of "floating" bus slots, i.e. event sources
that are bound to the lifetime of the event object itself, and thus
don't require an explicit reference to be kept.
sd-resolve: add "floating" resolve queries

Same story as for sd-bus and sd-event: allow passing NULL to store query
in in which case the query is freed automatically.
sd-bus: always keep slot reference while dispatching callback

Also, make sure we automatically destroy reply callbacks that are
floating.